President Obama himself jumped into the fray on Tuesday, offering a thorough denunciation of trickle-down economics and Rep. Paul Ryan's budget, which he linked to Mitt Romney, his most likely Republican challenger for the presidency.

Obama called the Ryan plan "thinly veiled social Darwinism" and slammed Romney for supporting it.

Obama also echoed the broad themes of economic fairness and shared responsibility he advanced during a speech in Kansas last fall and again at the State of the Union in January.

And Obama again pushed for the so-called Buffett Rule, which calls for higher taxes on some of the richest Americans.

Until Tuesday, Obama had been restrained in his specific criticisms of the Republican candidates.

"It's still primary season for the Republican Party; they're gonna make a decision about who their candidate will be," Obama told reporters at a press conference on Monday. "I will cut folks some slack for now because they're still trying to get their nomination."

Biden has been more specific -- and caustic -- calling out Republican presidential hopefuls by name during his swing state tour, and attacking them on everything from the auto bailout to Medicare, Social Security, corporate tax policy and trade.

Romney -- now on the fast track to his party's nomination -- has received the most attention from Biden. In Ohio, the vice president called out the former Massachusetts governor for opposing the auto bailout.

"Our friends on the other side ... said we would make auto companies 'wards of the state,' " Biden said. "Governor Romney was more direct -- let Detroit go bankrupt."

Of course, Romney's position was more nuanced. The candidate argued for a managed bankruptcy at the time of the bailout, and continues to insist that was the ideal course of action.

But the idea that the struggling automakers would have attracted enough private investment to stay afloat has been met with skepticism from many experts.

Speaking to members of the United Auto Workers in Ohio, Biden devoted a few hundred words to hammering that idea home.

"Any honest expert will tell you in 2009, no one was lining up to lend General Motors or Chrysler any money or for that matter to lend money to anybody," Biden said. "That includes Bain Capital. They weren't lining up to lend anybody any money either."

General Motors has since reported record profits, and the White House appears eager to make the case that its efforts were instrumental in saving the industry -- a direct line of attack on Romney.

Romney spokeswoman Andrea Saul said the administration "has done more to devastate the middle class than any in modern history." She cited unemployment, depressed home values and high gas prices.

"With that kind of record, it's no surprise that the Obama White House has taken to attacking a proven job creator like Mitt Romney," Saul added.

On other issues, the Obama-Biden campaign has been attacking Romney through Ryan, the budget committee chairman who has endorsed Romney. In Florida, Biden focused much of his speech on the Ryan budget, and linked that blueprint to Romney at every available opportunity.

"Romney supports cut, cap and balance, which is yet another demonstration that there is no daylight between Governor Romney and the Republican leaders on the most important issues facing this country," Biden said. "And not even Romney's Etch A Sketch can change that."

Rick Santorum and Newt Gingrich, meanwhile, have drawn only glancing attacks from the White House in recent weeks.

With Election Day still months away, perhaps the biggest variable in the campaign rhetoric equation is the economy itself.

The major stock indices went on a tear in the first quarter, and the unemployment rate dipped to 8.3%. Another reading is scheduled to be released on Friday.

A continuation of the unemployment trend would be welcome news for the White House. But there are reasons to think the recovery is not as strong as the robust jobs numbers would indicate.

Economic growth, for example, continues to lag. If growth were to slow in the second half of the year, as it did in 2010, the White House would be in a difficult spot.